WASHINGTON, March 20 (Reuters) - The Pentagon Inspector General said on Wednesday that it would investigate a complaint that acting Pentagon chief Patrick Shanahan, a former Boeing executive, violated ethical rules by allegedly promoting Boeing while in office.

Last week Citizens for Responsibility and Ethics in Washington, a watchdog group, filed a complaint with the Inspector General saying that Shanahan had appeared to violate the ethical rules by "promoting Boeing in the scope of his official duties at the Department of Defense (DOD) and disparaging the company's competitors to his subordinates."

"The Department of Defense Office of Inspector General has decided to investigate complaints we recently received that Acting Secretary Patrick Shanahan allegedly took actions to promote his former employer, Boeing, and disparage its competitors," said Dwrena Allen, a spokeswoman for the Inspector General.

Prior to taking over as acting Pentagon chief earlier this year, he was the deputy defense secretary.

Shanahan joined Boeing in 1986 and spent more than three decades there, working on the 737 and 787 Dreamliner. He was also the president and general manager of Boeing Missile Defense Systems and worked on the Apache, Chinook and Osprey military aircraft.

"Acting Secretary Shanahan has at all times remained committed to upholding his ethics agreement filed with the DoD," said Lieutenant Colonel Joe Buccino, a Pentagon spokesman.

"This agreement ensures any matters pertaining to Boeing are handled by appropriate officials within the Pentagon to eliminate any perceived or actual conflict of interest issue with Boeing," Buccino said.

During a Senate Armed Services Committee hearing last week, Shanahan said he would support an investigation by the Inspector General. (Reporting by Idrees Ali; Editing by Dan Grebler)